1. Field of the Invention
The present invention relates to an air compressor for use in an air piping system through which compressed air is supplied to air brakes, air suspension systems, automatic doors and the like.
2. Description of the Related Art
Generally speaking, an air piping system is constituted of an air compressor 1 and an air tank 2 linked by piping via an air dryer 3 and a check valve 4, as shown in FIG. 6. In the system, air is taken in and compressed by the air compressor 1, temporarily stored in the air tank 2 after being dehumidified with a drying agent in the air dryer 3 and then supplied to air brakes or the like via piping that is connected to the outlet-side of the air tank. Also connected to the air tank 2 via the piping is a governor 5, the exit side of which communicates with the unloader valves on the intake side and the outlet side of the air compressor 1 so that, when the pressure in the air tank 2 rises to a specific level, the governor 5 enters the open state, to operate the unloader valves, thereby setting the air compressor 1 in a decompression state (unload state). Moreover, when the pressure in the air tank 2 drops under a specific level, the governor 5 enters the closed state and is reset to its initial state, to start the compression function of the air compressor 1.
The air compressor disclosed in Japanese Unexamined Patent Publication No. H2-32875 is an example of this type of air compressor. This air compressor features a residual pressure forming valve which blocks off the outlet-side passage when the unload valve provided in the intake-side passage is closed, in order to suppress, as much as possible, any negative pressure in the cylinder during an unload and to prevent oil from being drawn up into the cylinder. According to this invention, with the residual pressure forming valve blocking off the outlet passage, discharge of the air inside the cylinder during an up-stroke of the piston is inhibited, to maintain the pressure in the cylinder at a high level, and since, even when the piston enters the down-stoke, the pressure in the cylinder does not become extremely negative, preventing the pressure in the cylinder from becoming negative.
However, in the example quoted above, since there is no air supply to the cylinder during an unload, if the unload time is long, the air in the cylinder overflows into the crank chamber through the gap between the cylinder and the piston, reducing the pressure in the cylinder, which may then become negative.
Furthermore, since the air that is thus released contains oil, carbon and the like in the cylinder, there is a likelihood of these substances, i.e., oil, carbon and the like, adhering to the sliding area of the residual pressure forming valve (outlet-side unloader valve), which may cause operational failure. Also, there is a likelihood of the valve body making a fluttering movement due to the pressure wave of the outlet pressure of the outlet air and becoming worn.